This invention involves highly sensitive optical detection/imaging devices designed for use in low light level applications or applications involving relatively high ambient optical noise. For such applications, a goal is to limit the effects of radiation propagating obliquely to axis of the device. This off-axis radiation includes ambient optical noise, as well as target light scattered by the entrance conduit of the device itself. A solution to this problem has been to provide a device having a window or conduit whose periphery absorbs off-axis radiation. This solution involves the provision of a peripheral region of absorbing medium such as black glass. In some applications, it has been found preferable, in lieu of a solid black absorbing region, to provide a peripheral region of glass having the same composition as a transparent core, and containing light absorbing fibers extending more or less radially of the axis of the conduit. Details of such a filtering conduit are disclosed in co-pending patent application Ser. No. 497,865, U.S. Pat. No. 4,533,210.
Prior conduits of this class have been manufactured as a simple cylinder of transparent medium completely surrounded by a thick layer of absorbing medium (black glass.) In such a conduit the absorbing medium is further extended radially at the outer face to serve as a contact stop along the periphery of the aperture of the frame of the detecting device. At the same time this extension provides a seat for the sealing of the conduit or faceplate to the frame of the optical imaging device. The primary light detecting component, usually a light-sensitive semi-conductor wafer, is sealed to the inner surface of the conduit and extends out over the absorbing layer as well.
The shoulder of the outer extended portion of the completed filter, which is to be aligned and sealed with a seat on the device frame, must be carefully polished to assure a consistently secure seal. The inner surface of the conduit must also be precisely polished to assure consistent bonding of the semi-conductor wafer. The polishing and sealing of the shoulder and the application of the wafer have hitherto produced inconsistent bonds. It has been found that some of the inconsistency is due to the relative difficulty of polishing the material making up the absorbing region, as compared with clear glass. These and other difficulties are also related to the differing coefficients of thermal expansion between absorbing glass and clear glass. Although these coefficients can be matched for a narrow temperature range, it is difficult to match them both for the operating temperature of the device and also for the temperature required for manufacture. The difficulty of polishing the absorbing medium makes the attainment of a proper seal at the shoulder of such material difficult. Furthermore, the inner surface of the conduit, where the detecting material is located, is inconsistently polished, producing inconsistent bonding of the semi-conductor wafer. The present invention obviates these and other difficulties of the prior art devices in a novel and unobvious ways.
It is, therefore, a primary object of the present invention to provide a faceplate for an optical imaging device which displays off-axis filtering capacity while allowing precise polishing at critical sealing surfaces.
Another object of the present invention is to provide a conduit for an optical detection device which displays off-axis filtering capability while presenting a homogeneous surface for the application of the semi-conductor detection element.
A further object of the present invention is the provision of a faceplate incorporating an off-axis filter which minimizes the need for careful matching of the coefficients of thermal expansion of the clear and absorbing regions respectively.
Another object of the present invention is to provide an off-axis filter which is simple in construction, economical to manufacture, and capable of a long life of useful service with a minimum of maintenance.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.